Optical glass fibers are frequently coated with two superposed photocured coatings. The coating which contacts the glass is a relatively soft, primary coating. The outer, exposed coating is a much harder secondary coating that provides desired resistance to handling forces, such as those encountered when the fiber is cabled.
The coating of optical glass fibers with photocured coating compositions, usually using ultraviolet light, is well known today. Photocuring compositions are selected because of their rapid cure speed. Faster cure speed is generally desirable to increase the production of optical glass fibers.
Coatings produced from conventional compositions including (meth)acrylate-terminated polyurethanes are much too hard to be utilized as primary coatings and exhibit poor adhesion and resistance to microbending, especially at low service temperatures. When a polyether acrylate monomer having a low glass transition temperature (T.sub.g) is added to these compositions in an amount sufficient to provide adequate flexibility, the water resistance and adhesion of the coating are usually reduced which is undesirable.
It is desirable to further increase the cure speed of the photocuring composition while retaining the capacity of the cured primary coating to adhere to the glass fiber surface and to resist water absorption.